[PATCH] Remove unnecessary unbind in LDAP search+bind mode

Hi!

Comments in src/backend/libpq/auth.c [1] say:
(after successfully finding the final DN to check the user-supplied
password against)
/* Unbind and disconnect from the LDAP server */
and later
/*
* Need to re-initialize the LDAP connection, so that we can bind to
* it with a different username.
*/

But the protocol actually permits multiple subsequent authentications
("binds" in LDAP parlance) over a single connection [2].
Moreover, inspection of the code revision history of mod_authnz_ldap,
pam_ldap, Bugzilla, and MediaWiki LDAP authentication plugin, shows that
they've been doing this bind-after-search over the same LDAP connection for
~20 years without any evidence of interoperability troubles.

(mod_authnz_ldap and pam_ldap are listed in the PostgreSQL documentation as
examples of other software implementing this scheme. Bugzilla and MediaWiki
are the original patch author's motivating examples [3])

Also it might be interesting to consider this note from the current
revision of the protocol RFC [4]:
"The Unbind operation is not the antithesis of the Bind operation as the
name implies. The naming of these operations are historical. The Unbind
operation should be thought of as the "quit" operation."

So, it seems like the whole connection re-initialization thing was just a
confusion caused by this very unfortunate "historical" naming, and can be
safely removed, thus saving quite a few network round-trips, especially for
the case of ldaps/starttls.

[1]
https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/libpq/auth.c;h=bc0cf26b122a1b28c20fe037ec851c0e99b1ffb6;hb=HEAD#l2603
[2] https://www.rfc-editor.org/rfc/rfc4511#section-4.2.1
[3]
https://www.postgresql.org/message-id/4c0112730909141334n201cadf3x2e288528a97883ca%40mail.gmail.com
[4] https://www.rfc-editor.org/rfc/rfc4511#section-4.3
--
Best regards,
Anatoly Zaretsky